Occlusive vascular disease of the brain, heart and lower extremities is the primary cause of morbidity and mortality in the US. Native (pre-existing) collaterals interconnect adjacent arterial trees and function as critical bypass vessels if obstruction occurs in one of the trees. Attention over the past 25 years has been on mechanisms mediating outward remodeling of collaterals in ischemic disease. However, until our work over the last three years, nothing was known about what controls the number and diameter (extent) of these vessels. We have shown that collateral extent in murine brain and other tissues varies widely due to genetic polymorphisms, resulting in a greatly increased risk-severity for ischemic injury of brain and other tissues in individuals having low collateral extenta finding recently confirmed in humans. We have discovered the major genetic locus, Candq1, responsible for most (70%) of this variation. We now seek to identify the polymorphic gene underlying this locus and its downstream effector pathway that regulate collateral formation. Aim I will fine-map Candq1 in a congenic strain-set that we have partly completed generating in preliminary studies: We will introgress informative partitions of the Candq1 locus of the C57BL/6 high-collateral strain into the BALB/c strain that has few-to-no collaterals in brain and other tissues. These congenic lines will then be phenotyped for collateral extent in brain and skeletal muscle, followed by functional analysis in a model of ischemic stroke in the positive congenic line(s) (+CNG) that most tightly partitions the causative region of Candq1. Aim II-A will additionally narrow the candidates by determining gene expression in the +CNG using RNA-Seq across Candq1, and also across the whole genome to identify potential downstream effectors. Aim II-B will further refine the candidates and targets, plus determine if a variant in cis/trans regulatory element underlies Candq1, using whole-genome chromatin mapping for active DNA regulatory sites. Aim I and II will thus identify the highest priority candidates that Am III will test for causality using in vivo gene targeting, cellular analyses, and functional outcomein stroke. This project will identify the primary driver gene that controls formation of the native collateral circulation and is responsible for its wide variation. Defining the gene underlying Candq1, and its downstream signaling effectors, will also pinpoint targets to develop as therapies aimed at inducing new collaterals to form in adults with few and/or who are at risk for or have suffered acute stroke or other cerebral ischemic conditions and diseases. Identifying this major allele that when polymorphic impairs collateral formation may also provide a genetic marker for use in these same patient groups.